1. Field of the Invention
This invention is directed to an apparatus and method for imaging a radiation intensity distribution of a source of x-ray and/or gamma-ray radiation.
2. Description of the Related Art
Several conventional devices and techniques using reflective optics (e.g., mirrors) or Fresnel zone plates exist for x-ray microscopy at radiation energies below two kilo-electron volts (keV). At radiation energies above two keV, the performance of x-ray reflective optics devices and techniques becomes poor because mirror reflectivity is relatively small for photons at these energies, and hence the length of the optical surfaces required to achieve grazing incidence reflection increases prohibitively. As a result, most reflective optics systems are unable to obtain spatial resolution below several tens of micrometers at radiation energies above two keV. The performance of devices and techniques using Fresnel zone plates also degenerates at relatively high radiation energies (i.e., above two keV) because the required thickness of a radiation-opaque material forming such Fresnel zone plates, increases at higher radiation energies while the required spacing of the radiation-opaque material decreases. Therefore, the Fresnel zone plates are required to have relatively thick regions of radiation-opaque material which are spaced relatively close together, a structure which is difficult to manufacture. Also, Fresnel zone plates must be designed for a relatively narrow range of radiation energies, a requirement which can limit the use of devices and techniques which employ Fresnel zone plates.
Other conceivable devices and techniques for imaging a source of x-ray or gamma-ray radiation might use coded aperture imaging in which a single mask has multiple apertures or pinholes placed between the source and a position-sensitive radiation detector. The detector records a transform of the source image which can be inverse-transformed to reconstruct an image of the radiation intensity distribution of the source. However, this method will not provide imaging at scales much finer than the finest scale measurable with the position-sensitive radiation detector, so that the position resolution of the source is typically limited to several tens of micrometers.
Other conventional devices and techniques for imaging a source of x-ray or gamma-ray radiation have been applied to radio-astronomy applications and use interferometers which include pairs of antennae along various base lines which are used to extract Fourier components. Also, with respect to x-ray radiation, modulation collimators and other designs utilizing arrangements of grids have been employed. However, these telescope arrangements are only suitable for applications in which the source to be imaged is relatively distant from the telescope arrangement. Accordingly, these telescope arrangements are not suitable for an x-ray or gamma-ray radiation microscope.
To summarize, the devices and techniques described above are not suitable for microscope applications producing relatively fine spatial resolution (e.g., as low as a few microns) for radiation energies above about one-tenth keV. Therefore, these conventional devices and techniques fail to meet the demands of applications such as inertial confinement fusion (ICF) experiments in which a target compressed to less than 100 microns in size radiates copious x-rays above two keV for a short time. Also, these conventional devices and techniques are inadequate for medical applications requiring relatively fine spatial resolution of sources which emit radiation at energies above two keV.